Applicants claim, under 35 U.S.C. xc2xa7119, the benefit of priority of the filing date of Dec. 23, 1999 of a German patent application, copy attached, Serial Number 199 62 278.7, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.
1. Field of the Invention
The invention relates to a position measuring system, having a scale and a scanning unit, which can be moved in relation to the scale in the measuring direction.
2. Discussion of Related Art
A first category of known devices for absolute position determination includes an absolute track on the scale side for coarse position determination, as well as several incremental tracks for the incremental position determination at higher resolution. In connection with such measuring systems for absolute position determination, reference is made, for example, to FIG. 6 in U.S. Pat. No. 5,252,825. Known serial pseudo-random codes can be employed for absolute position determination, for example. The further incremental tracks customarily include incremental graduations with graduation periods which are stepped with respect to each other, starting with a coarse incremental graduation period and up to a fine incremental graduation period. The finest incremental graduation period finally determines the maximum resolution of such a measuring system. Appropriate processing of the scanning signals of the different tracks is required during measuring operations in order to have the desired absolute position available at the output side. The several tracks extend on the scale parallel with each other in the measuring direction. Rotary, as well as linear measuring arrangements on the basis of this principle are known in this connection. Accordingly, different widths of the scales are therefore required, depending on the number of incremental tracks. In the same way, several detector elements per track are required for the scanning unit, which is movable with respect to the scale. With a correspondingly large number of incremental tracks, a voluminous structure results as a whole. For example, in case of a rotary system, a correspondingly large diameter of the graduation disk is required. Thus, problems sometimes arise under restricted installation conditions.
Furthermore, difficulties can arise if a metal strip is used as the scale carrier in the described systems. Erroneous measurements thus result if such a metal strip is not aligned exactly parallel with the measuring direction, and the various graduation tracks are twisted around an axis which is located perpendicular to the graduation plane. In this case processing the signals from the various tracks, and the absolute position determination from these signals can sometimes become problematical.
Furthermore, a multitude of incremental tracks require a large illuminated area on both sides of the scale, i.e. a relatively low illumination strength per unit of area exists on the scanned scale. The results of this are scanning signals with small signal amplitude, which are therefore susceptible to failure.
However, a second category of known devices for absolute position determination includes merely a single incremental track arranged parallel adjacent to an absolute track with a serial absolute coding. In this connection, reference is made to DE 195 05 176 A1, for example. Although the above-mentioned problems in connection with the structural size are avoided in such a variation, a clearly reduced resolution in the course of the position determination results in comparison with the above discussed systems. If, on the other hand, the resolutions of the incremental track and the absolute track are selected to be too different, problems result in particular in the synchronization of the signals from the different tracks.
Moreover, with such systems the use of metal strips for the scale is not without problems, since again wrong measurements can occur with the synchronization of the signals from the incremental track and the neighboring absolute track in case of metal strips which are not oriented parallel with the measuring direction. Basically the always necessary synchronization of the signals from the track with the highest resolution with the signals from the track with the next highest resolution is most critical here. Errors in this synchronization result in particular in cases, were twisting of the scale occurs around an axis which is oriented perpendicularly in relation to the scale surface. Such twisting as a rule occurs along thin metal strips, since they cannot be cut straight enough because of their lack of stiffness.
But the above discussed problems do no only occur in the described categories of absolute position measuring with a serially coded absolute track. Similar problems also exist in connection with rotary position measuring systems which, besides a high resolution incremental track, furthermore have an incremental track which provides an output signal with a single signal period in the course of one revolution.
It is therefore an object of the present invention to indicate a position measuring system of compact structure, which will provide several incremental signals of different resolution, if possible, from scanning an incremental track.
This object is attained by a position measuring system that includes a scale having an incremental track, which extends in a measuring direction and has an incremental graduation with two different graduation periods. A scanning unit moves in relation to the scale in a measuring direction, the scanning unit has an incremental signal detection arrangement, wherein the incremental signal detection arrangement generates incremental signals with a first coarse signal period and generates incremental signals with a second finer signal period.
The position measuring system in accordance with the present invention thus permits the generation of at least two incremental signals of different resolution while scanning only one incremental track.
A variation of the position measuring system of the invention, which in particular is suitable for absolute position determination, only includes two separate tracks. One of the two tracks is embodied as an absolute track and in the course of scanning provides coarse absolute position information. However, the second one of the two tracks, i.e. the incremental track, comprises an incremental graduation with at least two different graduation periods, and in the course of scanning provides at least two separate incremental signals with different signal periods. It is therefore possible to derive at least two incremental signals of different resolution from a single incremental track without the structural size of the total system becoming too large.
Moreover, the position measuring system in accordance with the present invention has been shown to be relatively indifferent to possible tilting of the scanning unit with respect to the scanned scale. With the above discussed categories of absolute position measuring systems in particular, the above mentioned synchronization of the signals from the track with the highest resolution with the signals of the track with the next highest resolution is now less critical, since these signals are obtained from a single, or common, track. This means that it is also possible to employ metal strips as the scale carrier. Because of this indifference to tilting or twisting of the scanning unit with respect to the scale, extreme demands are no longer made on mechanical components, such as machine guideways etc., which are employed in connection with the system in accordance with the invention.
It should be furthermore pointed out that in the case of optical scanning, a smaller illuminated field is sufficient for signal generation as a whole because of the reduced lateral dimensions of the two tracks. This in turn results in a greater illumination strength per unit of surface, and therefore a greater signal strength, or reduced susceptibility to failure.
Although embodiments of the position measuring system of the present invention will be described in what follows, each of which is based on optical, or photoelectric, scanning, it should be pointed out here that the present invention can of course also be carried out in connection with other physical scanning principles. It should also be mentioned that the system of the invention can be designed as a linear, as well as a rotary position measuring system.
It is of course also possible to provide, besides the exemplary embodiments of absolute position measuring systems explained in detail in what follows, alternative position measuring systems. For example, a rotary system with only a single incremental track is conceivable, whose scanning provides a first coarse incremental signal with one signal period per revolution, and a second, high-resolution incremental signal, etc.
Further advantages of the present invention ensue from the subsequent description of exemplary embodiments by means of the attached drawings.